The present invention generally relates to optical telecommunications and more particularly to an optical switching device that switches wavelength-multiplexed optical signals between a plurality of optical input ports and a plurality of optical output ports.
With the development of optical telecommunications, there is a demand for an optical switching device that carries out the switching of a wavelength-multiplexed signal between a plurality of input optical fibers and a plurality of output optical fibers. In such an optical switching device for switching wavelength-multiplexed signals, each of the input optical fibers carries a wavelength-multiplexed signal containing therein a plurality of optical signal components having respective wavelengths, while each of the output optical fibers are provided in correspondence to one of the wavelengths forming a channel of the wavelength-multiplexed signal. Such an optical switching of the wavelength-multiplexed signals is not only useful in the optical telecommunications but also in parallel processing computers wherein a plurality of processors are connected with each other by a common optical bus for exchanging data in the form of optical signals.
In the conventional art of optical switching, it has been practiced to convert optical signals once to electric signals, such that the desired switching of the optical signals is made in a digital switching device by switching the electric signals thus obtained. After switching, the electric signals are converted back to optical signals. Such a construction is naturally complex and has a drawback in that a very large scale digital switching device has to be provided.
On the other hand, there is a proposal to construct an optical switching device that carries out a switching of the optical signals without converting the same to electric signals. For example, Fallahi, et al. proposed an optical switching device in which a plurality of optical fibers are coupled to a diffraction grating, which acts as a spectroscope, by a number of optical couplers (Fallahi, M. et al., IEEE Photonics Technology Letters, vol. 15, no. 7, July, 1993, pp. 794-797). However, the foregoing optical switching device requires a large number of optical fibers cascaded by Y-shaped optical couplers for multiplexing the optical signals and thus has a drawback in that the construction of the switching device becomes very large and extensive when there are a large number of channels in the wavelength-multiplex optical signal.
FIG. 1 shows the construction of a conventional optical switching device that uses an optical waveguide.
Referring to FIG. 1, the optical switching device cooperates with a plurality of processing units #1-#k each including a plurality of processing elements PE#1-PE#n, wherein each of the processing elements PE#1-PE#n receives and outputs an electrical signal via a line that forms a cable collectively designated by a numeral 10. Thus, in the illustrated example, a cable 10 is provided in correspondence to each of the processing units #1-#k, wherein each line in the cable 10 is connected to an electro-optical conversion interface I/F for converting the electric signal in the line 10 to an optical signal. The optical signal thus produced is then supplied to an optical fiber collectively designated by a numeral 11. Further, the optical signal supplied to the interface I/F via the optical fiber 11 is supplied to a corresponding processing unit after conversion to an electric signal in the interface unit I/F. Thereby, it should be noted that each of the interfaces I/F receives electric signals from a plurality of processing units and produce a wavelength-multiplex optical signal in which the foregoing electric signals are multiplexed with respective, different wavelengths. The wavelength-multiplex optical signal thus produced is then outputted to the optical fiber 11. Further, the interface I/F separates the individual electric signals by carrying out a demultiplexing of the wavelength-multiplex optical signal supplied thereto from the optical fiber 11.
The wavelength-multiplex optical signal thus produced is then injected to a glass slab 12 that provides an optical waveguide. In the illustrated example, the glass slab 12 is provided in a plural number in correspondence to the processing elements #1-#n, wherein each optical waveguide 12 carries a plurality of ports l.sub.1, l.sub.2, . . . l.sub.k for connection of the foregoing optical fibers. Thus, the waveguide 12 is injected with a plurality of wavelength-multiplex optical signals at the foregoing ports l.sub.1 -l.sub.k, wherein the optical signals thus injected propagate through the waveguide 12 by repeatedly causing reflections therein. Further, the optical signals thus propagated through the waveguide 12 are outputted to the optical fiber(s) 11 via one or more of the foregoing ports l.sub.1 -l.sub.k.
In such a so-called broadcast type waveguide, there occurs a diffusion in the optical signals propagating therethrough. Thus, there arises a problem in that the intensity or reception level of the optical signals decreases at the foregoing input/output ports. When such a decrease occurs in the reception level of the optical signals, the S/N ratio of the optical switching is deteriorated inevitably. Further, such a broadcast type waveguide lacks selectivity of wavelengths, and because of this, it is necessary to provide a mechanism in the interface unit I/F for separating an optical signal of desired channel from the wavelength-multiplex optical signals. While such a separation of particular signal components from a wavelength multiplex signal may be achieved by using a multilayer filter that selects a particular wavelength as a result of Bragg diffraction, it should be noted that such a multilayer filter is not suitable for use in the broadcast type waveguide, as the optical signals arrive at the input/output ports in such a broadcast type waveguide with various angles of incidence that range from 0.degree. to 90.degree..
It is also possible to separate a wavelength-multiplex optical signal into a plurality of optical signal components by providing a diffraction grating at an end of a single ridge waveguide that guides the wavelength-multiplex signal therethrough, such that the optical signal components thus separated travel through respective optical waveguides as proposed by Fallahi, M., op. cit. Such a construction, however, has a drawback noted previously in that it is necessary to cascade a large number of optical couplers in order to inject optical signals into the ridge waveguide to produce a wavelength-multiplex optical signal therein. Further, such an approach is difficult to be adopted in constructing a multiple bit optical switching device in which a plurality of wavelength-multiplex signals corresponding to multiple bit data are processed by respective ridge waveguides that are stacked with each other in correspondence to each bit.
Meanwhile, there is an optical bus of ultra-high speed computers that has a function similar to the optical switching device under consideration.
FIG. 2 shows the construction of such an optical bus proposed previously.
Referring to FIG. 2, the optical bus is constructed from an optical waveguide 21 having substantially parallel upper and lower major surfaces, wherein the waveguide 21 supports thereon a substrate 22 that in turn carries a plurality of parallel processors 23. The substrate 22 further carries light emitting elements 23a and photodetection elements 23b in correspondence to the plurality of processors 23, wherein each of the light emitting elements 23a injects the output of the processor 23 into the optical waveguide 21 in the form of optical signal. The optical signal thus injected is guided through the optical waveguide 21 by causing a multiple reflection between the upper and lower major surfaces and is detected by the foregoing photodetection element(s) 23b. In order to facilitate the injection as well as the detection of the optical signals, the optical waveguide 21 is provided with microlenses 21a in correspondence to the light emitting devices 23a and the photodetection devices 23b.
Thus, it will be noted that the optical waveguide 21 of FIG. 2 falls in the category of the broadcasting type waveguide and cannot avoid the problem of diffusion of the optical signals in the optical waveguide as well as the problem of decrease of the reception signal level associated with such a diffusion. Further, such a construction has a drawback in that it is difficult to provide wavelength selection function to the photoreception device.